1. Field of the Invention
The present invention relates to a pin platen mechanism for supporting and feeding a medium such as a continuous sheet, a continuous tablet or the like having sprocket holes therethrough.
2. Description of the Prior Art
A pin platen mechanism of this type, which is to be assembled into a printer or the like, includes sprocket wheels each having sprocket pins fitted in the medium sprocket holes, the sprocket wheels being respectively mounted on both ends of a platen serving to support the medium having the sprocket holes therethrough on the side of a printing surface, which sprocket wheels are then rotated integrally with the platen for thereby feeding the medium. If such sprocket pins are fixedly mounted on the sprocket wheels to project from the sprocket wheels at all times, the movement of a ribbon protector mounted on a carriage together with a print head will be disturbed. This limits the movement of the carriage.
To solve this difficulty of limiting the extent of movement of the carriage, a known mechanism, such as disclosed for example in the Japanese Patent Publication No. 36-22836, enables the sprocket pins to move into and out of the sprocket wheel by use of a cam member having a cam groove formed therein and combined with the sprocket wheel. The sprocket pin is withdrawn into the sprocket wheel by the cam groove when the sprocket pin is located on the side of the printing surface.
FIG. 4 is a side cross sectional view illustrating such a known pin platen mechanism which permits a sprocket pin to freely move into and out of a sprocket wheel, and FIG. 5 is a front cross sectional view illustrating a portion thereof.
First, as shown in FIGS. 4 and 5, designated at 1 is a platen, 2 is a central axle or shaft of the platen 1, 3 is a sprocket wheel, 4 are sprocket pins, 5 is a cam member, 6 is a paper guide, 7 is a locking ring, 8 are washers, 9 is a platen spring, 10 is a bearing, 11 is a gear, and 12 is a spring pin. These members constitute the pin platen mechanism and are supported on side frames 13 of a printer, one side frame being shown in FIG. 5.
In addition, designated at 14 is a bottom paper guide, and 15a and 15b are respective media such as continuous paper tablets or sheets, each media having sprocket holes through both sides thereof into which the sprocket pins 4 are fitted.
Designated at 16 is a pressure roller disposed to push the medium 15a or 15b against the platen.
Likewise, designated at 17 is a main shaft disposed parallel to the platen 1, and 18 is a carriage mounted on the main shaft 17, on which carriage 18 is mounted a print head 19 having a tip end surface adapted to face the platen 1. Further attached to carriage 18 via a bracket 22 is a ribbon protector 21 for preventing the medium 15a from being stained by an inked ribbon 20 set so as to pass through the tip end surface of the print head 19.
Designated at 23 is a slider mounted on the lower surface of the rear portion of the carriage 18, and 24 is a guide frame permitting the slider 23 to be engaged slidably therewith. The carriage 18 is adapted to reciprocate along the platen 1 axially thereof with the aid of a drive source (not shown) while being guided by the guide frame 24 and by the main shaft 17.
Respective parts of the pin platen mechanism will be described in further detail with reference to FIG. 6.
FIG. 6 illustrating a prior pin platen mechanism is an exploded perspective view. Opposite ends of central shaft or axle 2 are formed to provide portions 2a having opposite flattened surfaces and circular end portions 2b having respective stopper holes 2c formed therein.
Each sprocket wheel 3 has through the center thereof a mounting hole 3a adapted to fit complementarily on the respective flattened part 2a of the central shaft 2, as well as a plurality of holding or retaining holes 3b extending radially from the circumferential surface thereof toward the center thereof, and further has a stopper groove 3c provided circumferentially at one axial end thereof. In addition, each sprocket pin 4 includes a projection 4a formed on the base thereof.
Each ring cam member 5 has a substantially D-shaped annular cam groove 5a formed in one axial end thereof and fixing projections 5b formed on the opposite axial end thereof.
Each paper guide 6 has fixing holes 6a corresponding to the fixing projections 5b as well as a guide portion 6b extending from the outer periphery thereof at one axial end thereof, the guide portion 6b having therein a pin groove 6c and a notch portion 6d. Moreover, the paper guide 6 has on the opposite axial end thereof a plurality of circular arc-shaped first holder parts 6e and second holder parts 6f formed to define a cylinder, any one of the second holder parts 6f having on the inner surface thereof an anti-rotation projection 6g.
Furthermore, the first holder parts 6e are thicker and have larger outer diameters than the second holder parts 6f.
Each locking ring 7 has on the outer periphery thereof a lever 7a and a plurality of stopper pawls 7b and further has on the inner periphery thereof a locking projection strip 7c corresponding to the first holder parts 6e of the paper guide 6 and an unlocking groove 7d.
Each of the ends 2b of the central shaft 2 is rotatably supported in a respective bearing 10, in the peripheral surface of which is provided an anti-rotation groove 10c corresponding to the anti-rotation projection 6g of the respective paper guide 6, and in the vicinity of one end of bearing 10 is provided a blade portion 10a having anti-falling-off projections 10b.
Moreover, the side frames 13 are provided in confronting relation with each other, each of the frames 13 has a support part 13a notched into a U-shape and anti-falling-off holes 13b corresponding to anti-falling-off projections 10b of the respective bearing 10. Designated at 25 is a knockout pin.
In succession, assembly of these parts now will be described.
First, the base parts of the sprocket pins 4 are inserted into the respective holding holes 3b in the sprocket wheel 3 while the cam member 5 is fitted in the inside of the sprocket wheel 3 so as to permit the projections 4a of sprocket pins 4 to extend into the cam groove 5a. In addition, the paper guide 6 is brought into contact with the cam member 5 with the first holder parts 6e and the second holder parts 6f positioned outwardly of the cam member 5 with respect to the shaft center, and with fixing projections 5b fitted in fixing holes 6a. Thereafter, the locking ring 7 is fitted to the outer peripheries of the first and second holder parts 6e and 6f of the paper guide 6, and tip portions of the plurality of the stopper pawls 7b formed on the locking ring 7 are engaged in the stopper groove 3c in the sprocket wheel 3 All the members described above are thus integrally united.
The locking ring 7 is adapted to be rotatable with the stopper pawls 7b engaged in the stopper groove 3c, and a first holder part 6e is adapted upon engagement of the above members with each other to enter the unlocking groove 7d in the locking ring 7.
Then, these parts 3 to 7 integrally united as such are inserted onto the left end of the central shaft 2 of the platen 1, the mounting hole 3a in the sprocket wheel 3 is fitted on the flattened part 2a provided on the central shaft 2, and the platen spring 9 is mounted on the end part 2b of the central shaft 2 in a sandwiching relation between the washers 8. Thereafter, the bearing 10 and the gear 11 are respectively fitted to the end part 2b of the central shaft 2, and finally the spring pin 12 is inserted into gear 11 and the stopper hole 2c formed through the central shaft 2 to prevent the respective parts from being removed from the shaft.
On the other hand, for the right side of the central shaft 2, the sprocket wheel 3, cam member 5, paper guide 6, locking ring 7, and bearing 10 are likewise combined and mounted on the central shaft 2, and the knockout pin 25 is inserted into the stopper hole 2c to prevent these parts from being removed from the shaft.
After assembling the pin platen mechanism by mounting the respective parts on the right and left sides of the central shaft 2 as described above, the respective bearings 10 are pushed into the support parts 13a of the left and right side frames 13 and fixed thereon so as not to be rotatable with respect thereto, with the anti-falling-off projections 10a provided on the blades 10b of the bearings 10 respectively fitted in the anti-falling-off holes 13b formed in the side frames 13, whereby the pin platen mechanism is supported by the side frames 13.
The platen spring 9 in this state makes contact at opposite ends with a stepped portion formed in the interior of the bearing 10 and the end surface of the flattened portion 2a provided on the central shaft 2 via the washers 8 to press the central shaft 2 for absorbing any back lash produced in the platen 1 upon assembly thereof.
In addition, the anti-rotation projection 6g provided on the second holder part 6f of each paper guide 6 has thereupon been fitted in the anti-rotation groove 10c in the respective bearing 10 fixed non-rotatably on the side frame 13 as described previously to prevent the paper guide 6 from being rotated. Moreover, each cam member 5 has its fixing projections 5b fitted in the fixing holes 6a in the respective paper guide 6, and thus is also prevented from rotating, while being aligned with a linear part of the cam groove 5a oriented vertically on the side of the ribbon protector 21. Only the sprocket wheels 3 which holds the sprocket pins 4 are adapted to be rotatable together with the central shaft 2 and the platen 1 by permitting the mounting hole 3a formed in the sprocket wheels 3 to be fitted to the flattened portions 2a of the central shaft 2.
Furthermore, the right and left paper guides 6 are respectively mounted on the right and left sides of the central shaft 2 symmetrically thereof, and hence are positioned when assembled as described above so as to permit the respective notched portions 6d to correspond to the bottom paper guides 14 as shown in FIG. 4.
The operation of the above arrangement now will be described.
First, when the first holder parts 6e of the paper guides 6 are fitted into respective of the unlocking grooves 7d in the locking rings 7, the first holder parts 6e make slight contact with the bearings 10 inserted into the inside of the first holder parts or are separated therefrom by small gaps, and hence the sprocket wheels 3, cam member 5, and paper guides 6 integrally united with the locking rings 7 are axially movable. Thus, these members are moved to adjust for the width of the medium 15a or 15b, whereby the left and right paper guides 6 are positioned to align with both side edges of the medium 15a or 15b.
Thereafter, with the lever 7a of each locking ring 7 being rotated manually by a prescribed angle in a prescribed direction, the locking projection strip 7c provided on the inside of the locking ring 7 moves around the outside of the first holder parts 6e of the paper guide 6 to clamp the respective first holder parts 6e inwardly to thereby bring them into contact with the outer peripheral surface of the bearing 10, whereby the paper guide 6, the cam member 5, and the sprocket wheel 3 are fixed.
After this operation, when employing the medium 15a, such medium 15a is inserted between the sprocket wheels 3 and the guide parts 6b of the paper guides 6 from the back side of the platen 1, and the sprocket holes in the medium 15a are fitted to the tips of the sprocket pins 4 projecting externally from the sprocket wheels 3, while the medium 15a is wound around the platen 1 and is forced to pass between the platen 1 and the pressure roller 16 while being guided in a prescribed direction.
In addition, when employing the medium 15b, such medium 15b is inserted into the bottom paper guide 14 from below and is forced to pass through the notched portions 6d in the paper guides 6, and the sprocket holes in the medium 15b are fitted to the tips of the sprocket pins 4, while the medium 15b is wound around the platen 1 and is forced to pass between the platen 1 and the pressure roller 16 while being guided in a prescribed direction.
In this situation, the carriage 18 is moved axially of the platen 1 and the print head 19 is driven for printing on the medium 15a or 15b, and furthermore the medium 15a or 15b is advanced by one line for each axial printing movement.
This feeding of the medium 15a or 15b is effected by rotating the platen 1 and the sprocket wheels 3 integrally with the central shaft 2 through the transmission of rotary power of a line feed motor, etc., (not shown) to the gear 11. Thereupon, although the sprocket pins 4 are disengaged from the sprocket holes in the medium 15a or 15b in the vicinity of the pressure roller 16, other sprocket pins 4 are fitted into other sprocket holes in the vicinity of the pin grooves 6c in the paper guides 6 or in the vicinity of the notched portions 6d for thereby permitting the medium 15a or 15b to be fed in succession.
Moreover, although as the sprocket wheels 3 are rotated, the respective sprocket pins 4 are also rotated, since the cam members 5 are fixed as described previously, the projections 4a of the respective sprocket pins 4 freely fitted in the substantially D-shaped cam grooves 5a provided in the cam members 5 are moved according to the shape of the cam grooves 5a.
Since each cam groove 5a is substantially D-shaped as described above, its linear portion is arranged vertically on the side of the printing surface, when the projection 4a of a given sprocket pin 4 is moved along such linear portion, the tip end of the sprocket pin 4 is forced to withdraw into the holder hole 3b in the sprocket wheel 3 following the corresponding movement of the projection, and the tip end of the sprocket pin 4 is projected from the holder hole 3b toward the outside of the sprocket wheel 3 when the projection 4a passes beyond the linear portion.
Thus, even if the ribbon protector 21 is moved to the end surface of the platen 1 together with the print head 19 by the carriage 18 during the above printing movement or upon finishing of such printing movement, the sprocket pin 4 does not disturb the movement of the ribbon protector 21.
As described above, since the known pin platen mechanism shown in FIGS. 4 to 6 is adapted to cause the tip ends of the sprocket pins to withdraw into the retaining holes in the sprocket wheels on the side of the printing surface, the movement of the ribbon protector is not disturbed, and thereby the carriage, on which the ribbon protector is mounted, is permitted to be moved over a wider extent. However, since the tip ends of the sprocket pins are positively withdrawn into the holder holes on the side of the printing surface without fail, the number of the effective sprocket pins engaging the medium is reduced, thus resulting in a reduction of feeding capability of the mechanism as a whole.
Moreover, with a reduced contact area, as with a medium fed through the bottom paper guide, between the medium and the sprocket wheels, the reduction of the effective number of sprocket pins achieving feeding of the medium as described above is very disadvantageous. Furthermore, upon copying and the like the sprocket pins can become disengaged from the sprocket holes owing to swelling due to fixing needles, pastes, etc., for tying those media up into a bundle, thus resulting in difficulty in feeding of the media.